1. Field of the Invention
The invention pertains generally to fuel management system having an open loop calibration which includes provision for special condition calibrations and is more particularly directed to special condition calibrations for starting and warm up enrichment.
2. Prior Art
Electronic fuel schedulers or electronic control units for regulating the air/fuel ratio of an internal combustion engine are conventional in the art. These schedulers provide, from a calculation or electronic computation based upon the operating parameters of the engine, an air/fuel ratio that is considered substantially ideal for the instantaneous conditions sensed.
The "best" air/fuel ratio at which the engine will operate under a given set of operational conditions is normally a tradeoff between the competing factors of driveability, emissions, and fuel economy. It is generally understood that richer air/fuel ratios are better for power and driveability, a substantially stoichiometric air/fuel ratio the most desirable for emissions, and lean air/fuel ratios the calibration that gives the best fuel economy. The schedule of desired air/fuel ratios for the electronic control unit can be derived from empirical tests of emissions, driveability, and economy tests and may include areas where the one criterion is more important than the others.
For example, under urban or in city driving, conditions emissions are considered of importance because of the congestion of automobiles present in a small area and the amount of pollutants at these slow speeds while at highway or freeway speeds, economy would be the overriding factor of consideration. In addition, for passing or accelerations and to ease starting and warm up situations, power and driveability must be factored into scheduling.
Any number of the various engine parameters may be sensed to calibrate the schedule of air/fuel ratios, but the most advantageous method is to measure mass air flow or mass fuel flow and calculate the other from the schedule.
An air/fuel controller having a calibration based upon the speed of the engine and the density of the air as a measurement of mass air flow has been successfully provided by a U.S. Pat. No. 3,734,068 issued to J. N. Reddy on May 22, 1973. The disclosure of Reddy is hereby expressed incorporated by reference herein. Reddy discloses a base calibration pulse width that is a function of the RPM of the engine and manifold absolute pressure. The duration of the pulse width is used to regulate fuel flow to the engine based upon a schedule. This base calibration is an open loop control of the air/fuel ratio as the operating parameters of the engine are sensed by the controller and a control signal which is the fuel pulse duration is developed therefrom.
If the air/fuel ratio schedule from which the control signal is calculated or the engine environment to which it is applied is different from the optimum design system, then the controller will not perform as required. The difference in engine environments are generally either because of manufacturing tolerances that change the response of the engine, or, as occurs with all mechanical devices, the ageing factor which is difficult to schedule.
It is known in the art that to solve many of the problems faced by open loop fuel schedulers a closed loop integral controller may be effectively utilized. The controllers are termed "closed loop" because they sense the result of an actual air/fuel ratio change and develop a control signal based therein rather than calculate an air/fuel ratio change from a desired schedule as does the open loop controller. One of the most advantageous of these controller systems is based upon the bi-level output of an exhaust gas composition sensor which indicates whether a rich or lean air/fuel ratio charge has been combusted by the engine. The controller incrementally leans the air/fuel ratio during a rich indication of the sensor and incrementally enrichens the air/fuel ratio during a lean indication of the sensors, thereby causing the system to oscillate in a limit cycle about a desired air/fuel ratio. Illustrative of this type of controller is a U.S. Pat. No. 3,815,561 issued to Seity which is commonly assigned with the present application. The disclosure of Seity is hereby expressly incorporated by reference herein.
In addition to the speed-density calculation, the basic open loop calibration for mass air flow can and is generally corrected for special conditions. One of the more important of the special condition calibrations to the open loop schedule should be the starting condition where it is known that a much richer air/fuel ratio than normal is used to insure the quick and facile initial operation of the engine.
One fuel management system with a start calibration makes use of the cranking of the engine to develop an enrichment pulse for every engine revolution. The enrichment or length of the pulses is dependent upon engine temperature with substantial enrichment occurring at lower engine temperatures and decreasing to a minimum as the engine warms to operating temperature. A positive detection of the cranking condition, the starter solenoid engagement is generated to enable the pulses during its presence.
While this system produces the quick and facile starts it was designed to accomplish, it does create an emission problem during starter overrun. This time period is the short increment after the engine has started but before the operator recognizes the start has occurred by releasing the starter solenoid.
Even though relatively short in length, this period can cause significant HC, CO emission impact because the starting enrichment pulses are speed dependent and of a length designed to provide a substantially enrichened air/fuel ratio at cranking speeds of between 30-60 RPM of the engine. As the engine starts, its crankshaft rotational speed rapidly increases to approximately five to ten times cranking speed to where the engine can sustain operation at an idle range of 300-600 RPMs or fast idle (850 RPM) in the best case. During the worst case condition, and generally in the manner many operators start engines, the engine will be accelerated to a much higher RPM than idle before the operator recognizes a start and releases not only the starter solenoid but also the accelerator pedal which he has depressed fully in his effort to start the engine. It would be desirable to suppress the emissions caused by these high rotational speeds and the enrichened cranking pulse width without leaning out the fuel mixture before starting the engine.
The fuel management system described above further has a warm up calibration which is a function of both time and engine temperature. The temperature dependent portion provides enrichment during this period based upon temperature only with a greater enrichment provided at colder temperature and less enrichment at warmer temperatures. This enrichment which is somewhat less than the starting enrichment, provides a smooth transition between the start calibration and the operational or base calibration. Since it is dependent upon engine temperature, its magnitude will only be that which is needed for satisfactory driveability. The other component of the warm up enrichment calibration is time and temperature dependent. Initially, this component has a hold level enrichment value whose magnitude is based upon engine temperature. The level is a maximum at colder temperatures and decreases to a minimum at warmer temperatures. From this holding level, the enrichment level decays with a time constant to zero after a delay.
It has been found that this system provides unnecessary enrichment during the holding or delay time at higher engine temperatures that are close to operational conditions. It would therefore be desirable, if when sensing these conditions, to defeat the holding period and proceed directly to the decay portion of the time and temperature dependent component of the warm up calibration.
A further advantageous modification to a warm up calibration is one for load. When the engine temperature is cold, more enrichment for high loads is necessary than at the same load for a fully warm engine. Generally, a colder engine and a high load will necessitate the most enrichment and a light load with a fully warm engine, the least. A U.S. Pat. No. 3,971,354 issued to Luchaco et al, discloses a load dependent warm up enrichment circuit that provides enrichment as a function of MAP. The enrichment as a function of load provided by the Luchaco circuit is a modification of a temperature dependent warm up calibration and does not provide correction for a time dependent component such as that included in the present system. Further, Luchaco provides the most load enrichment in the power operating region of the MAP curve as seen in FIG. 4 of that reference and does not produce significant warm up load enrichment at MAP values below these portions of the MAP curve.
It has now been found that enrichment provided for warm up values at loads within the normal operating range (300-600 torr) of MAP values increases driveability and response of the engine. Moreover, a linearly increasing enrichment smooths the transisions between load values in this intermediate range where accelerations and decelerations are common.